Process valves in fluid flow lines are commonly operated by applying an external force to a spindle attached to the valve member. Because the spindle penetrates the valve body, a potential leakage path to the atmosphere is created. Although valve stem leakage is always sought to be minimized, leakage of some process fluids cannot be tolerated at all, as in the case of hazardous materials, fluids which are too valuable to lose in this manner and fluids which present unusually difficult maintenance problems when they escape.
Attempts to prevent valve stem leakage have been made in the past, resulting in various types of valve designs. In one type involving a sealed valve body, disclosed in U.S. Pat. No. 3,206,160, a ferromagnetic ball valve encased in a sealed housing is caused to move between open and closed positions by the magnetic fields produced by energizing coils which surround the housing. Although this arrangement provides a sealed valve, it suffers from other drawbacks. By limiting the ball to ferromagnetic material, the valve does not have applicability to environments in which the process fluid is hostile to the ball material. Further, the ball positioning technique is not as accurate as desired in view of the inevitable drift of the system due to heating of the coils and to the disclosed voltage divider arrangement.
Another example of a sealed valve is in U.S. Pat. No. 3,347,262, which discloses the use of a motor to rotate an outer cylindrically shaped magnet to cause rotation of an inner cylindrical magnet. Protuberances on a valve plug are contacted by the rotating inner magnet, causing the valve plug to open or close. This design does not have means for precisely positioning the valve plug, nor would the magnets be capable of developing the necessary torque required in many commercial process valve applications.
What is needed is an effective, reliable sealed valve that has no potential leaks to the atmosphere, which can deliver the high torque necessary in many process valve environments and which can be accurately controlled to position the valve at any predetermined point, whether fully open, fully closed or at an intermediate point between the two extremes.